The present invention relates to an optical module for use in a field of an optical communication, and more particularly, to an optical module including a collimator comprising an optical fiber and a lens.
In a field of an optical communication, when it is required to give an action to a light transmitted through an optical fiber, an optical device is inserted in an optical path. Since a light emitting from the optical fiber diverges, a collimator which converts a diverging light to a parallel light is used in order to efficiently introduce the light to the optical device. The collimator and the optical device are practically modularized to be used as the optical module.
FIG. 1 shows, as one example of the optical module, a filter module 200 which separates a light of a multiplexed wavelength in the optical communication of a wavelength multiplex system.
Now, a light having two multiplexed wavelengths xcex1, xcex2 enters a lens 121 from an optical fiber 111. The lens 121 is a gradient index rod lens, whose refractive index is distributed in a radial direction of a circular section perpendicular to an optical axis. When a lens length of the lens 121 is adequately designed, a diverging light entered from the optical fiber 111 is converted to a parallel light. A spectral separation optical filter 150 which is an optical device is arranged in contact with an end face opposite to a light receiving surface (an end face opposed to the optical fiber 111) of the lens 121. The optical filter 150 reflects the light having a wavelength xcex1 and transmits the light having the wavelength xcex2. The light reflected by the optical filter 150 is converged by the lens 121 and enters an optical fiber 113.
The light (wavelength xcex2) transmitted through the optical filter 150 enters a lens 122, is converged therein and then enters an optical fiber 112. Generally, the lens 121 and the lens 122 have the same characteristics. An optical system is integrated into one housing 170 to constitute the optical filter module 200.
A basic optical system of the collimator used for the optical module will be described in accordance with FIG. 2. Now, consider a case where point light sources 211, 213 are arranged on one focal surface 281 of a first convex lens 221, and on an optical axis 201 and at a position xcex94P separated from the optical axis 201, respectively, as shown in FIG. 2A. In view of geometrical optics, a light from the light source on the focal surface 281 is converted to a parallel light by the convex lens 221. However, unless the position of the light source exists on the optical axis 201, the direction of the parallel light inclines with reference to the optical axis 201 in accordance with the position xcex94P of the light source. When the parallel light beam enters a second convex lens 222 having the same focal distance as the first convex lens 221, images 212, 214 are formed at positions symmetrical with respect to the light sources 211, 213 regarding both the lenses 221, 222.
A traveling state of the light beam is the same as shown in FIG. 2B, even when the lenses are gradient index rod lenses 321, 322. The rod lenses 321, 322 are cylindrical, whose refractive indexes are distributed along radial directions from the cross sectional center. A refractive index distribution n(r) is ideally represented by the following equation:
n(r)=n0(1xe2x88x92(A/2)r2) 
wherein r is a distance from a center axis of the lens, n0 is a refractive index on a center axis of the lens, A1/2 is a refractive index distribution constant. A meandering period (pitch) P of the light beam in the gradient index rod lens is represented by P=2xcex/A1/2. Here, for simplification, the gradient index rod lens having a lens length of 0.25 P is illustrated. In the lens of 0.25 P, a light generated from the point source light on the end face is converted to a parallel light beam and then emitted.
A case where point light sources 311, 313 are arranged on one end face of a first convex lens 321 on an optical axis 301 at a spaced-apart position xcex94P from the optical axis 301 respectively is considered. However, unless the position of the light source exists on the optical axis, the direction of the parallel light inclines with reference to the optical axis depending upon the position xcex94P of the light source in the same manner as in the convex lens. When the parallel light enters a second convex lens 322 having the same pitch as the first lens 321, images 312, 314 are formed at the symmetrical position of the light source relative to both lenses 321, 322.
Generally, when two lenses having limited effective diameters are arranged on the same optical axis, the optical axis inclines when a distance L between the lenses increases, consequently there is a case where a part of the light can not enter the second lens.
In the case of the optical filter module 200 shown in FIG. 1, since each of optical fibers 111, 113 has a limited diameter, at least one optical fiber is arranged apart from the center axis of lens. The light entering and emitted from the optical fiber arranged apart from the center axis of lens has predetermined angle with respect to the center axis at another end face of the lens.
For example, when the optical axis of the optical fiber 111 is coincident with the center axis of the lens 121, the incident light toward the lens becomes parallel relative to the center axis of the lens at the emitting end of the lens and is emitted perpendicularly with reference to the end face of the lens. In this case, however, since a part of the light which is reflected at the end face of the lens returns to the optical fiber 111, the reflected return light is undesirable for the optical communication. When in manufacturing the optical module, it is not always easy to frequently adjust the position of the incident light to perpendicularly emit the light from the end face of the lens, even if a reflected return light problem may not be considered.
Actually, as shown in FIG. 1, the end face of the optical fiber 111 and the face of the lens opposed to the optical fiber are normally formed to incline relative to the optical axis in order to prevent the reflected return light. In addition, even when the light enters from the lens face inclined with respect to the optical axis, the outgoing light from the lens inclines with respect to the optical axis.
A method will be considered to incline the center axis of the lens 122 relative to the optical axes of the optical fiber 111 and the lens 121 in order to converge the light having a given angle relative to the center axis of the lens 121 and to introduce thereto with a low loss. According to the method, the center axes of lenses 121 and 122 are arranged with a predetermined angle in the optical filter module 200 in FIG. 1. In this case, it is necessary for an inside diameter of a sleeve 160 to be largely formed than that of a glass holder 144 to keep a sufficient space for adjusting the angle.
Now, the assembly process of the optical filter module 200 will be described below.
First, the collimator 201 including two optical fibers 111, 113, the lens 121 and the optical filter 150 is assembled. Distal ends of two optical fibers are inserted into a capillary 131 having, for example, two holes and secured thereto with an adhesive, and then the end face of the capillary 131 is polished. The capillary with optical fibers 131 is inserted into a glass holder 140 capped with a cylindrical metal tube and secured thereto with the adhesive.
The optical filter 150 is adhered to the end face of lens 121. The lens 121 is inserted into the glass holder 142, and secured thereto with the adhesive. In this case, the light having the wavelength xcex1 enters the lens 121 from the optical fiber 111, and the capillary 131 and the lens 121 are adjusted so that the reflected light from the optical filter 150 becomes a maximum amount. A core adjustment is performed with respect to three directions, an x1-axis, a y1-axis, and a z1-axis. Where, the x1-axis and the y1-axis are perpendicularly oriented relative to the center axis of lens 121, and are mutually oriented to two directions at a right angle as well, and the z1-axis is oriented to the optical axis direction of lens 121.
Then, the collimator 202 including the lens 122 and the optical fiber 112 is assembled. The lens 122 and the optical fiber 112 secured to the capillary 132 are inserted into a common metal holder 144 and are adjusted in the z2-axis and secured thereto. The z2-axis is oriented to the optical axis of the lens 122.
Next, two sets of collimators 201, 202 are combined together and the optical filter module 200 is assembled. It is necessary that a z1-axis and a z2-axis are obliquely set with respect to the optical axis direction. At this time, the X-axis, the Y-axis, and oblique angles xcex8x, xcex8y are adjusted so that the light amount coupled to the optical fiber 112 becomes the maximum value by means of introducing the light having the xcex2 wavelength from the optical fiber 111. Where, the X-axis and the Y-axis are perpendicularly oriented relative to the center axis (Z-axis) of the housing 170 and are mutually oriented at a right angle, the oblique angle xcex8x is a vertically oriented angle with respect to the center axis (Z-axis), the oblique angle xcex8y is a horizontally oriented angle with respect to the center axis (Z-axis). The core adjustment of the center axis 170 in the center axis direction may be performed but there is a case to omit this procedure because of a troublesome operation.
A small diameter beam is used for the optical filter module 200. Because of that reason, when an angle displacement between two collimators 201, 202 exceeds 0.02 degrees, the module 200, therefore, can not be used due to an increase of the light loss. Mechanical stability of the optical filter module 200 must be maintained in the operating temperature range, for example xe2x88x9220 to 70xc2x0 C. Because of this, a solder is generally used to fill a relatively large space and secure the lens and the optical fiber with high reliability. Although resins are easy to be handled, they are not suitable because of shrinkage in curing and of a large coefficient of thermal expansion.
In the optical filter module 200, glass holders 140, 142, 144 capped with the sleeve 160 and the metal tube are secured by the solder 180. The solder is poured into the sleeve 160 from a solder injection port 182. The housing 170 and the sleeve 160 are secured by the resin 190 for protecting the optical fiber.
Because the assembly of an existing optical module requires the core adjustment process frequently, there is a problem to take a long time for it. In addition, a displacement of the adjusted core position is easy to occur in a high temperature state where a solder is melted and in a cooling back thereof, since the solder is used. Because of this, there are problems that yield becomes low and the productivity is decreased.
Furthermore, the resin is easy to deteriorate with heat in solder melting since the resin is used for securing the optical fiber and the capillary, and the capillary and the lens. The deteriorated resin may be a cause of a problem in terms of long-term reliability of the optical module. A special treatment is required such that the holder of the capillary is capped with the metal tube and gold plating is formed on the outside of the metal tube and the inside of the lens and the sleeve or the like because of the securing with the solder. In addition, port processing for pouring the solder into the sleeve is required.
Furthermore, because two collimators are obliquely secured to each other, there is an inconvenience that the whole is upsized and a large space is required therein.
As another optical filter module, an optical filter module 511 including an optical system as shown in FIG. 3, for example, is proposed by the present inventors. An optical filter module 511 includes a predetermined integrated first lens 512 and a second lens 513, the optical filter 514 arranged between both lenses, two optical fibers 515, 516 arranged on an incident side of the first lens 512, and one optical fiber 517 arranged on an emitting side of the second lens 513 so that each of optical axes becomes substantially coaxial.
Two optical fibers 515, 516 are secured so that optical fiber axes become parallel substantially. Each of the optical fibers 515, 516 is separately arranged on both sides of the optical axis C with the same distance xcex94P1 deviated therefrom. The optical fiber 517 is arranged on the same side of the optical axis C as the optical fiber 516 with the same distance xcex94P1 deviated therefrom. The same lens having a focal distance f is used as the first lens 512 and the second lens 513, a distance between lenses is set by 2f and the optical filter 514 is arranged at a focal position of the lens 512.
When the optical filter module 511 emits a light having a wavelength region transmitting through the optical filter 514 from the optical fiber 515, the light is collimated by the first lens 512 and transmits through the optical filter 514, and the transmitted light is converged by the second lens 513 and is adapted to enter the optical fiber 517. In addition, when the optical filter module 511 emits the light having the wavelength region reflected by the optical filter 514 from the optical fiber 515, the light is collimated by the first lens 512 and reflected by the optical filter 514, and the light is converged by the first lens 512 and is adapted to enter the optical fiber 516.
The following core adjustment operations (a) to (d) are required to assemble the optical filter module 511 shown in FIG. 3. The core adjustment operation required for the optical filter module 511 assembly will be described below.
The light propagating in the optical fiber is not completely confined in the core, and the light in the bottom part of the intensity distribution seeps in a cladding. In the case of a single mode optical fiber, an outgoing light is a Gaussian beam showing an intensity distribution similar to Gaussian distribution-shaped (the distribution that intensity is strong in the center of the light flux, and becomes gradually week at a peripheral portion) in the plane perpendicular relative to the optical fiber axis. In other words, the outgoing light from the optical fiber 515 (Gaussian beam) is collimated by the first lens 512 but does not become the parallel light between both lenses 512, 513 and a beam waist is formed therebetween. The light converged by the second lens 513 does not converge at one point but the beam waist is formed.
(a) A core-adjustment operation to make an end face of the optical fiber 516 coincide with a position of a beam waist of a light wherein the light having a wavelength region reflected by a filter 514 is emitted from the optical fiber 515, then the light reflected by the optical filter 514 is converged by the first lens 512 and enters the optical fiber 516.
In this operation, the reflected light by the optical filter 514 emitted from the optical fiber on a reflecting port side is monitored, and the cores of the optical fibers 515, 516 are adjusted in the X, Y, and Z direction so that the light amount of the reflected light becomes maximum amount.
Further, the core adjustment operation wherein the light having a wavelength region transmitted the optical filter 514 is emitted from the optical fiber 515, the light transmitted through the optical filter 514 is converged by the second lens 513 then a position of the beam waist of the light entering the optical fiber 517 and the end face of the optical fiber 517 is adapted to coincide.
In this operation, the transmitted light of the optical filter 514 emitted from the optical fiber 517 is monitored, and the core of the optical fiber 517 is adjusted in the X, Y, and Z directions so that the light amount of the transmitted light becomes maximum amount.
(b) A core adjustment operation to make coincide with a mode field diameter and a beam waist diameter of each of optical fibers 515 to 517 respectively.
In this operation, the same lens is used for both lenses 512, 513. Therefore, a position of the beam waist of the light emitted from the optical fiber 515 and a position of the beam waist of the light entering the optical fiber 517 become symmetry with reference to the optical axis C, and a diameter of the mode field and a diameter of the beam waist of the optical fibers 515, 517 wherein the core adjustment operation (a) was performed are coincident with each other. In addition, a position of the beam waist of the light emitted from the optical fiber 515 and a position of the beam waist of the light entering the optical fiber 516 become symmetry with reference to the optical axis C, a diameter of the mode field and a diameter of the beam waist of each of optical fibers 515, 516 wherein the core adjustment operation (a) was performed are coincident with each other. xe2x80x9cA mode field diameterxe2x80x9d is a diameter of a beam having an intensity of 1/C2 of peak of a Gaussian distribution-shaped intensity distribution.
(c) A core adjustment operation to make an optical fiber axis of each of optical fibers 515 to 517 and a principal ray of the beam coincide with each other.
In this operation, a vertical oblique angle xcex8x with respect to the optical axis C and a horizontal oblique angle xcex8y with respect to the optical axis C are adjusted respectively for each of optical fibers 515 to 517.
(d) A core adjustment operation to avoid an eclipse of beam on the way.
In this operation, a vertical oblique angle of xcex8x with respect to the optical axis C and a horizontal oblique angle xcex8y with respect to the optical axis C are adjusted respectively for both lenses 512, 513.
For the optical filter module 511, it takes time to perform the core adjustment operation of all (a) to (d) to obtain the required insertion loss of the optical filter module and takes long time to assemble the optical filter module. Therefore, productivity of the optical filter module becomes low and cost therefor will be elevated.
It is an object of this invention to provide an optical module which satisfies a required insertion loss and can be easily assembled with high productivity.
To achieve the above object, the present invention provides an optical module. The optical module includes a first optical fiber and a first lens. The first lens is optically coupled with the first optical fiber to receive an incident light from the first optical fiber and to convert the incident light into a parallel light. An optical device receives the parallel light and performs predetermined optical processing on the parallel light. A second lens receives a transmitted parallel light from the optical device and converges the transmitted parallel light to produce an outgoing light. A second optical fiber is optically coupled with the second lens and receives the outgoing light. An optical axis of the first lens and an optical axis of the second lens are substantially coincident with each other. An optical axis of the first optical fiber and an optical axis of the second optical fiber are substantially parallel with each other and substantially parallel to the optical axes of the first and the second lenses.
A further perspective of the present invention is a method for assembling an optical module. The optical module includes a first optical fiber, a first lens, optically coupled with the first optical fiber, for receiving an incident light from the first optical fiber and converting the incident light into a parallel light; an optical device for receiving the parallel light and performing predetermined optical processing on the parallel light, a second lens for receiving a transmitted parallel light from the optical device and converging the transmitted parallel light to produce an outgoing light, and a second optical fiber, optically coupled with the second lens, for receiving the outgoing light. The method includes making an optical axis of the first lens and an optical axis of the second lens coincide with each other, securing the optical device to a predetermined position between the first and the second lenses, securing the first and the second lenses so that a distance between a center of a light emitting face of the first lens and a center of a light receiving face of the second lens opposed to the first lens becomes a predetermined value, arranging the first and the second optical fibers in parallel with optical axes of the first and the second lenses, introducing a light having a predetermined wavelength and transmitting through the optical device into the first lens from the first optical fiber, adjusting at least either of a relative position between the first optical fiber and the first lens and a relative position between the second optical fiber and the second lens in the same direction as an optical axis of the lens and in two directions perpendicular to the optical axis thereof so that a light amount which enters the second optical fiber becomes larger than a predetermined value, and securing a whole optical module by keeping the adjusted conditions.
A further perspective of the present invention is a method for assembling an optical module. The optical module includes a first optical fiber; a first lens, optically coupled with the first optical fiber, for receiving an incident light from the first optical fiber and converting the incident light into a parallel light, an optical device, for receiving the parallel light and performing predetermined optical processing on the parallel light, a third optical fiber, optically coupled with the first lens, and having an axis in parallel with the optical axis of the first optical fiber and separated as much as a predetermined distance, a second lens, for receiving a transmitted parallel light from the optical device and converging the transmitted parallel light to produce an outgoing light, and a second optical fiber, optically coupled with the second lens, for receiving the outgoing light. The first lens converges the parallel light reflected by the optical device, produces a reflected outgoing light, and provides the reflected outgoing light to the third optical fiber. The method includes making an optical axis of a first lens and an optical axis of a second lens substantially coincident with each other, securing the optical device to a predetermined position between the first and the second lenses, securing the first and the second lenses so that a distance between a center of a light emitting face of the first lens and a center of a light receiving surface of the second lens opposed to the first lens becomes a predetermined value, arranging the first and the second optical fibers in parallel with optical axes of the first and the second lenses, introducing a light having a predetermined wavelength reflected by the optical device and a light having a predetermined wavelength transmitting through the optical device separately or concurrently into the first lens from the first optical fiber, adjusting a relative position between the first optical fiber and the first lens in the same direction as an optical axis of the lens and in two directions perpendicular to the optical axis thereof so that a light amount which enters the third optical fiber becomes larger than a predetermined value, adjusting a relative position between the second optical fiber and the second lens in the same direction as an optical axis of the lens and in two vertical directions to the optical axis thereof so that a light amount which enters the second optical fiber becomes larger than a predetermined value, and securing a whole optical module keeping the adjusted conditions.
A further perspective of the present invention is a method for assembling an optical module. The optical module includes a first optical fiber; a first lens, optically coupled with the first optical fiber, for receiving an incident light from the first optical fiber and converting the incident light into a parallel light, an optical device, for receiving the parallel light and performing predetermined optical processing on the parallel light, a third optical fiber, optically coupled with the first lens, and having an axis in parallel with the optical axis of the first optical fiber and separated as much as a predetermined distance; a second lens, for receiving a transmitted parallel light from the optical device and converging the transmitted parallel light to produce an outgoing light; and a second optical fiber, optically coupled with the second lens, for receiving the outgoing light. The first lens has a focal point with a focal distance f, converges the parallel light reflected by the optical device, produces a reflected outgoing light, and provides the reflected outgoing light to the third optical fiber. The method includes arranging the first lens and the second lens so that an optical axis of the first lens and an optical axis of the second lens are substantially coincident with each other, arranging the optical device at a predetermined position between the first lens and the second lens so that a displacement amount from a focal point of the first lens is within xc2x125% of a focal length of the first lens, coupling the first and the third optical fibers with the first lens so that each of a vertical oblique angle xcex8x and a horizontal oblique angle xcex8y with respect to the optical axis of the first lens is within 0.2 degrees, and coupling the second optical fiber with the second lens so that each of a vertical oblique angle xcex8x and a horizontal oblique angle xcex8y with respect to the optical axis of the second lens is within 0.2 degrees.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.